This document explores the skill of ECMWF’s seasonal forecast, to predict dry spells. Bladiebla Have leadtime 1 to 6, where 1 is the month the forecast was released. However, forecast only released on 13th of the month.
This analysis
The figure below shows for how many months the % of ensemble members was below 180 mm. The 180 mm was the threshold set based on overlap of dry spells and monthly precipitation analysis.
We can see that the number of months lowers as the percentage increases, which is expected. While patterns differ slightly between lead times, the pattern is rather similair.
To set the threshold of % of members, we look at the observed data and count how many months had <=180mm. This were 21 months. We then choose the % for each leadtime which has closest to 21 months forecasted <= 180mm
After setting the % of members, we get a list of forecasted months for which we would have triggered. Now we can compare these with the observed months with less than 180 mm of precipitation.